1. Field
The present disclosure relates to optical amplification, and particularly relates to a photo-parametric amplifier that uses the properties of solid state detectors for very low noise amplification of weak, pulsed, high frequency optical signals.
2. Description of Related Art
Many biomedical instruments require the detection of very faint optical signals. For example the fluorescence of a single dye molecule that is coupled to an antibody may yield information about where a particular protein is located within a cell, but it produces very few photons/sec even with strong illumination. Many instruments detect such weak light sources with photomultiplier tubes (PMTs), where a photo-cathode converts a photon into a free electron, which is then accelerated by an electrical field to a dynode. Typical PMTs use 10 dynodes and provide a current amplification by a factor of about one million.
The internal gain is the significant advantage for PMTs over solid state photo detectors which have no built-in gain mechanism and which have to rely on external electronics to amplify the photo current to usable levels. PMT optical amplifiers outperform those based on photodiodes (solid state photodetectors) in terms of overall sensitivity and signal to noise ratio, even though photodiodes are actually much better at sensing light. The quantum efficiency (i.e., the probability for one photon to generate one electron) of a photodiode often exceeds 80% while it is rare that a PMT has a quantum efficiency that approaches 30%.
Photodiodes are extremely linear devices, e.g., the current output is strictly proportional to the light input for over 12 decades while a PMT barely maintains linearity over 3 decades. Photodiodes are very rugged devices that are not harmed by exposure to high light levels, while PMTs are fragile and easily destroyed by exposure to room light levels while powered on. Photodiodes are available that operate well with IR light, while PMTs can barely detect light in the near IR spectrum. Photons with wavelength in excess of 1 micrometer do not have enough energy to free an electron, thus there are no practical PMTs that can cover the 1-2 micrometer wavelength range, while solid state detectors exist that can operate up to 10 micrometer wavelengths. Finally, PMTs are large, fragile, expensive and require high voltages to operate while solid state detectors are small, rugged and relatively inexpensive. In summary, the photo diodes outperform PMTs in most aspects other than built-in amplification by a wide margin.